The present invention relates to an apparatus for forming images and is applied to, for example, a full color photocopier that forms images on a photosensitive recording medium through the process of exposure and development.
In an apparatus for forming images like a conventional full color photocopier, images are formed by exposing a photosensitive medium to light transmitted through or reflected by an original. A light source generally used for exposure is a halogen lamp or a fluorescent lamp, which is turned on during exposure and is then turned off.
In the photocopier, a larger quantity of light shortens the exposure time, and photocopies may be made at a higher speed. But even the largest quantity of light emitted from a halogen lamp is not sufficient for high-speed photocopy.
When a photosensitive material with a lower sensitivity than a semi-conductive sensitized material is used in the photocopier, higher luminous intensity is required for sufficient exposure. For example, when a photosensitive recording medium coated with sensitive microcapsules is used in the copying machine, a more efficient light source than the halogen lamp is required.
To solve the problems described above, a discharge lamp such as a metal halide lamp or a high-pressure sodium lamp is used as a light source. The metal halide lamp provides approximately three times as much light intensity as the halogen lamp with an identical electric power, and thus allows a higher copying speed. The metal halide lamp also gives higher luminous intensity, thus allowing a photosensitive material with a lower sensitivity to be exposed sufficiently.
The conventional image forming apparatus with the metal halide lamp, however, has the following problems. It takes a long time to stabilize the metal halide lamp or the discharge lamp after lighting. Hence, for the practical purpose, the metal halide lamp must not be turned off but remain `on` though it requires large electric power. The long time for lighting of the lamp also causes the temperature to rise on the surface of the lamp, makes the color temperature of emitted light unstable, and also makes the tone of light variable. Stable images of a desirable color or tone are thus not obtained.